Such actuator systems usually consist of a motor having a rotary output whose motion is translated by means of a ball and screw mechanism to the reciprocating action of an actuating element. The latter element is normally connected to a flight control surface on an aircraft to thereby position the same with respect to an airfoil member.
In many aircraft the control of such surfaces as flaps, spoilers, empennages and rudder assemblies are also assisted by the use of braking devices. For example, one known embodiment uses a pair of rachet wheels and pawl devices in conjunction with a disc brake or load bearing wheel member disposed between the rachet wheels and mounted on the ball and screw shaft for rotation therwith. In this arrangement the torque between the disc brake wheel and the rachet wheel (which ever wheel is being racheted by the load bearing wheel) is proportional to the load on the screw shaft (and hence the actuator for the flight control surface), and the rachet wheel is caused to rotate by the frictional engagement between the disc brake wheel and the rachet wheel, thus providing a continuous, load-dependent friction or damping counterforce to the load on the screw shaft. The teeth of each rachet wheel cooperate with a pawl mechanism to allow rotation in one direction only, the teeth of each wheel also being configured oppositely to the teeth of the other wheel so that its respective rotation direction corresponds to the angular direction (forward or reverse) taken by the ball-screw shaft. Consequently, as torque is established between one of the rachet wheels and the disc brake wheel stability is enhanced during movement of the actuator shaft and ultimately the flight control surface, so that fluttering, wobbling, or lost motion is dampened out.
With this arrangement heretofore described, usually some sort of back-up or redundant braking system, such as that described in the U.S. Pat. No. 4,603,594, is available in case of an undetected failure occuring in the primary system. Such detection-free actuator systems usually require, therefore, additional motor and gear train duty cycles and the like, or such other redundant or secondary systems that would normally add to their cost and weight.